The present invention relates generally to sewing machines, and more particularly to a speed control circuit capable of generating a precision speed control signal over a wide range of speeds.
In a prior art speed control circuit of a sewing machine, as shown and described in U.S. Pat. No. 4,386,301 issued to Neki et al, a tachogenerator produces speed pulses at a frequency proportional to the rotational speed of a sewing machine motor having a brake-and-clutch arrangement. A variable frequency divider is provided to divide the frequency of the speed pulse by a factor which is in turn controlled by a reference speed signal supplied from a speed setting setting device, so that the frequency divider produces output pulses at a frequency which is a submultiple of the original frequency. The interval between successive ones of the output pulses is then measured and applied to a transfer circuit which transforms the measured interval according to a predetermined transfer function to control the brake-and-clutch arrangement with the transformed interval. Because of the closed loop operation, the measured interval approaches a constant value after the motor has attained a steady speed regardless of its value. While this prior art is advantageous for implementing the transfer circuit with a microprocessor, the motor speed tends to exihibit a stepwise variation corresponding to one pulse interval when the frequency dividing factor is changed in response to the resetting of the reference speed. Although the stepwise variation could be reduced by theoretically increasing the number of speed pulses generated for each revolution of the motor, there is a practical limit to the number of pulses to be generated.